Photohardenable elements have a support and a layer of photosensitive material, which is photopolymerizable or photocrosslinkable, applied thereon. Photohardenable elements can be used to prepare flexographic printing plates. Such elements generally comprise (1) an addition polymerizable nongaseous ethylenically unsaturated monomer, (2) a photoinitiating system activated by actinic radiation, and (3) an elastomeric or thermoplastic-elastomeric polymeric binder comprising polymerized conjugated diene monomers. Photohardenable compositions such as these frequently contain plasticizers to increase the softness and flexibility of the developed flexographic plates.
Processes for making photohardenable elements and producing flexographic printing plates therefrom are well known in the art. Photohardenable elements are typically prepared by mixing the monomer, the photoinitiator, and the binder, usually at an elevated temperature. A plasticizer is then added to the mixture and the resulting mixture is then formed into a sheet structure by several known methods such as solvent casting, hot pressing, calendering and extrusion. Generally, the mixture is formed into a sheet structure by using an extrusion and calendering process. Once the photopolymer composition is in the form of a sheet, the element can then be used to make a printing plate by first exposing the element imagewise to actinic radiation. The areas of the photohardenable layer which are exposed to the radiation are photopolymerized or photocrosslinked and thereby become less soluble in developer solvents. The unexposed or unpolymerized areas of the element are subsequently washed off using a suitable developer solvent. There can be additional post-development treatments such as detackification and post-exposure.
There are problems associated with making a photohardenable element using the above-described process. For example, it is often difficult to add plasticizer materials to the other components of the photopolymerizable composition if the plasticizer used is a highly viscous material. The plasticizer cannot be added directly to the other components prior to mixing because it causes agglomeration of the solid materials. Similarly, when the plasticizer is added after mixing has begun, as in a later stage of an extruder, it is difficult to pump the plasticizer from its drum into the mixing unit. Residual plasticizer remaining in the drum also presents a disposal problem. Thus, for environmental reasons, as well as ease of operation, an alternative process is needed.
European patent application EP 0 442 358, published Aug. 21, 1991 describes a process in which the liquid constituents of the photohardenable mixture are premixed with the elastomeric binder, which is granular, until they are absorbed by the binder. The mixture is then dusted with a powder which is not soluble in the liquids. This process is disadvantageous because an additional powdered material is introduced into the photohardenable composition. Such powdered material adds dust, is difficult to handle, and may adversely impact the properties of the composition.
There are also problems associated with the process to prepare the flexographic printing plate from the photohardenable element. For example, when exposing the element imagewise to actinic radiation, it is frequently difficult to obtain the proper exposure time. The exposure time must be long enough to polymerize small areas, i.e., the highlight dots, so that these areas are not washed away with the developer solvent. At the same time, the exposure time must not be too long or fine lines, which are intended to remain unexposed and be washed out, i.e., the reverse lines, will polymerize and not wash out. In either case, if the exposure is too short or too long, the resolution and detail of the final plate is inadequate. It is particularly difficult to obtain the proper exposure with plates in which the elastomeric binder is a copolymer of isoprene, e.g., a styrene-isoprene-styrene block copolymer. It has been found that the photoactivity is greater and the exposure latitude even lower in flexographic plates containing these binders.
It is therefore desirable to have a process for preparing photohardenable elastomeric elements and flexographic printing plates which overcomes the above-described disadvantages.